In material removing machining of workpieces, cutting bits embodied as round plates or bits and retained in a chip removal or material removal tool, hereinafter called machining tool for short, are often used as elements such as the cutting tool. They are distinguished by a circular cutting edge that is concentric to their longitudinal center axis. The round bits are rotationally symmetrical with respect to the longitudinal center axis and have a securing opening that is coaxial with that axis. In the installed state, a securing screw extends through this opening, thereby retaining the round bit in a suitable receptacle of the machining tool. The round bit is often fixed in such way that its longitudinal center of axis is at an acute angle to the direction of relative motion between the tool and the workpiece. This means that the cutting edge, as it is fed, is not at a right angle to the direction of motion, and reaction forces which can have force components acting circumferentially of the round bit are exerted upon the round bit during machining.
Such a force component generates a torque acting on the round bit that can cause torsion or twisting of the round bit unless the force is diverted entirely into the machining tool. However, depending on the direction of rotation, such torsion of the round bit can cause the securing screw to loosen, or to become so tight that it can no longer be loosened at all. A loosened round bit can damage the remainder of the milling tool head and must absolutely be avoided. Unintended tightening and seizing of the securing screw must also be prevented.
In the industry, round bits intended for milling tool heads are known that have an annular basic body and form fitting or interlocking coupling between the round bit and a milling body. The round bit has a planar bottom face and an annular chip face that is spaced apart from the bottom face on the opposite side of the basic body. A curved side face, between the cutting and bottom faces, extends around the circumference of the annular basic body. The side face is provided with plane surfaces adjacent to one another, which are arranged obliquely both to one another and to the bottom face, so that the side face is bounded polygonally at the bottom face, and the chip face is bounded by the circular cutting edge. The receptacle provided in the milling tool head is also provided with a polygonal wall, which is contacted by the side wall of the round bit, so that the round bit. is in form fitting or interlocking engagement with the body of the milling tool head.
Torque acting upon this round bit is translated by the plane surfaces supported against the wall into a radial force acting upon the securing screw, which can cause the round bit to come out of adjustment in the milling tool head.
German Utility Model G 93 05 518.3 U1 discloses a similar round bit, which on its side face, here called its circumferential. face, has a total of eight plane surfaces that act as positioning faces for positioning the round bit in the milling tool head. The round bit is secured against torsion by the positioning faces as they contact corresponding bearing faces of the miller; once again, the torque on the round bit can produce radial reaction forces.
In the polygonal cutting bit described in European Patent disclosure EP 0 432 340 B1 as well, lateral positioning faces serve to position the bit and secure it against torsion.
Moreover, German Patent DE 36 42 514 C1 discloses a polygonal cutting bit with curved cutting edges, a total of three of which are provided in a side face extending between a plane top and a plane bottom face parallel to it. The bearing or positioning faces are curved in a plane parallel to the bottom face, and the radius of curvature is greater than that of the associated remaining side face. The positioning faces are braced against suitably curved faces provided on the milling tool head. Reaction forces that are produced are absorbed by a securing screw, which extends through a central opening in the cutting bit, and are diverted into the milling tool head.
Cutting bits of this kind are also known from European Patent Disclosure EP 0 307 949 A1; here the bearing or positioning faces are planar, however, and contact corresponding, likewise planar seat faces of a ball miller. Once again, as in the milling tool heads described above, wedge forces acting on the cutting bit can arise at the bearing or positioning faces.
A fundamentally different way to secure round bits against torsion is disclosed by German Utility Model G 92 01 113.6 U1. It describes a substantially frustoconical cutting bit with a plane top and a side face resting on a conical jacket. The side face is adjoined by a bottom face that is provided with grooves extending radially to the axis of symmetry of the round bit. The grooves are distributed uniformly, so that any two grooves adjacent to one another form the same angle as any other two adjacent grooves. A central opening leads through the cutting bit to receive a securing screw. A machining tool provided for receiving the cutting bit has a receptacle with a seat face into which a pin is inserted that engages one of the grooves when the cutting bit is secured to the tool.
Instead of the grooves, small indentations bounded on all sides and receiving the pin may be provided on the bottom face.
A separate alignment pin must be provided on the machining tool. Moreover, in this cutting bit the bottom face serving as the bearing face, which is exposed to considerable pressure per unit of surface area during use of the milling tool head, is reduced in size by the recesses. As a result, the force acting on the seat face associated with the bottom face and provided on the base body of the milling tool head is unevenly distributed.
European Patent Disclosure EP 0 545 669 discloses a similar cutting bit, which has a cylindrical base body with a central securing opening. The base body is provided on its bottom face with a total of four grooves extending radially outward, one of which cooperates with a corresponding alignment pin provided in the seat face of the machining tool.
Once again it is necessary to provide a separate alignment pin in the machining tool, and the result once again is an uneven distribution of force over the seat face.
Moreover, it is known in the industry to provide recesses in the side face of round bits, in order to secure them in form fitting or interlocking fashion against torsion in a machining tool. The recesses cooperate with corresponding protrusions provided on a lateral support surface of the machining tool. If the lateral support surface is to be able to perform its supporting task, these protrusions required high-precision machining.
Finally, it is known in the industry to secure cutting bits without a central opening to a basic tool body by means of clamping claws, which engage a central indentation provided in the top of the cutting bit. By means of the clamping claw, the cutting bit is pressed against a seat face provided on the base body of the tool and is retained thereon.